1. Field of the Invention
This invention relates to a single sideband modulator capable of fabrication on a single semiconductor chip.
2. Brief Description of the Prior Art
Single sideband (SSB) modulators have been employed in numerous communication, radar and ECM systems for many years. Most modern systems employ solid state modulators which have been designed using both active (FET, BJT, etc.) and passive (diodes) components in a variety of circuit configurations, ranging from multiple single balanced structures to double-double balanced structures. In all practical applications, unwanted sideband suppression and carrier rejection are prime performance considerations.
With the continual miniaturization of circuits, it becomes desirable to provide single sideband modulators which are capable of fabrication on a single semiconductor chip and, preferably, along with other circuit components on the same chip and which are also capable of operation in the microwave range. This requires that all of the components of the modulator be capable of fabrication on a single chip and that associated circuitry be compatible therewith, i.e., be capable of fabrication on the same chip. It is also necessary that there be no baluns on the chip in order that a monolithic configuration be attainable. In addition, when operation is at microwave frequencies, it is readily apparent that considerable improvement in circuit performance is attainable due to reduced size and superior device matching in such miniaturized monolithic circuits.
It is well known that when a pair of signals, as, for example, a carrier of frequency f1 and a modulating signal of frequency f2 are mixed in a mixer or modulator, the output thereof is generally a combination of f1, f2, f1+f2 and f1-f2. The sum and difference signals are referred to as the side bands. In single sideband transmission, it is desired to operate with only one of the sum (f1+f2) or difference (f1-f2) signals.
In the prior art, in order to obtain single side band, there is a requirement of at least two mixers that are separate and there is the outside balance to obtain the proper phase relationships whereby one side band can be cancelled.
A further problem is encountered when the frequency of the sidebands is only slightly different from that of the carrier frequency. It is readily apparent that, as the frequency of the sidebands approaches the frequency of the carrier, the ability to filter out the sideband signal relative to the carrier signal or vice versa, using standard filter circuits, becomes extremely difficult. In the case of, for example, a 7 gigahertz carrier with a sideband removed therefrom by 20 kilohertz, electronic filtering using filter circuits becomes virtually impossible. Prior art circuits therefrom have required the use of a balun with each mixer as well as external circuitry to obtain the single sideband. It is therefore necessary that other means be used to provide the required filtering action.